This invention relates to color effect pigments, and more particularly to the use thereof in coatings to provide a goniochromatic and/or reflective finish. Goniochromaticity is the effect of perceived color varying as the angle of illumination or observation varies.
Goniochromatic or reflective pigments are used, for example, in automotive coatings, decorative coatings, plastics pigmentation, printing inks (security inks in particular), and cosmetics. Their optical effect results from the directional reflection of light from predominantly sheetlike particles that are metallic or that have a refractive index contrast. According to the nature of the pigment particles, the pigments are known as metallic effect pigments (for example, aluminum, zinc, copper or alloys thereof) or interference pigments (for example, based on titanium dioxide-coated mica such as muscovite, phlogopite and biotite).
As a result of the incident light being reflected directionally by the predominantly sheetlike particles, effect pigments that are oriented, for example in a coating, exhibit goniochromaticity; that is, their perceived color (lightness and/or hue and/or chroma) varies with the angle of illumination or observation.
Interference pigments may comprise a single platelike layer, or a multilayer structure. The color perceived is affected by, for example, interference in the thin layer or layers, and optionally also by absorption by a chromophore (an organic moiety that absorbs wavelengths in the visible and/or UV ranges) or a color center. A color center is an electron hole pair that results from a lattice defect in a crystalline solid state material and which absorbs wavelengths in the visible and/or UV ranges. Interference, with or without absorption, results in a multiplicity of hue variations that are dependent on the thickness of the thin layer or layers.
Conventional metallic effect pigments of the type comprised of elemental metal are subject to oxidation. Oxidation is undesirable because it can reduce the reflectivity of the metallic particles, and because it can result in generation of gases within a coating composition that is in the liquid state. The surfaces of the metallic particles are sometimes coated with organic or metal oxide materials or subjected to other treatments in order to reduce the oxidation or to alter the appearance provided by the pigment. However, none of these approaches have been able to stop the oxidation totally, and some of the surface treatments tend to reduce reflectivity of the particles.
Metallic effect pigments are dense compared with many of the materials in which they are used; for example, typical aluminum pigments have a specific gravity of about 2.7 compared with specific gravities in the range of about 0.8 to 1.5 for a typical ink or coating composition. It is therefore a challenge to maintain such pigments uniformly suspended in the compositions.
Other color effect pigments rely upon interference phenomena generated by the presence of one or more thin films to develop high chroma (i.e., purity of color). Available pigments of this type generally use only one to five thin film layers of materials that have high refractive index contrast. Examples of this type are generally known and include, but are not limited to, metal oxide-coated silicatic (including mica) and metallic pigments. Because of the nature of the thin film materials used in these prior art interference pigments, their specific gravities typically exceed about 2.5, often substantially higher.
The difference in density between typical prior art color effect pigments and the liquid media within which they are suspended is undesirable. Although these pigments can be dispersed throughout the lower density medium by various methods known to those skilled in the art, the higher density pigments can settle due to the action of gravity. This settling is undesirable because it results in an inhomogeneous pigment distribution that must be corrected by time-consuming re-dispersion of the pigment.
It would be desirable to have available color effect pigments whose densities are closer to those of the liquid media of the compositions in which they are used. It would also be desirable to enhance the goniochromatic effects and/or the directional, high reflectivity of such pigments.